Postnatal changes in an alpha subunit isoform, alpha(S), of Na+,K(+)-ATPase in the submandibular gland of rats

The alpha and alpha(+) isoforms of Na+,K(+)-ATPase were isolated from the kidney and brain of rats and purified. Their antisera were raised to analyze the alpha isoforms in rat tissues. We found that the submandibular gland (SMG) contains a new immunoreactive alpha subunit isoform, designated alpha(S) in this report, in addition to alpha identical with those found in the kidney or brain. The new alpha(S) strongly reacted with anti-alpha-antiserum but to a much lesser extent with anti-alpha(+)-antiserum. The alpha(S) had a slightly lower molecular weight (approximately 90,000) than the brain and kidney alpha isoforms. Various fractions of SMG tissues were added to the SMG microsomes and incubated in order to test whether or not the alpha(S) is formed artificially; no increase of alpha(S) was observed by these treatments, suggesting that the alpha(S) was not the product formed from alpha during the preparation of microsome sample, but was rather a protein originally present in the SMG. The alpha(S) protein was not detected in the SMG of 2- or 5-week-old rats, but it gradually increased in rats older than 8 weeks, reaching the maximum in 30-week-old animals. The Na+,K(+)-ATPase activity in the SMG increased concomitantly with the increase of alpha(S), indicating that Na+,K(+)-ATPase comprising alpha(S) also shows enzyme activity; it is speculated that alpha(S) may have some unique and unknown function(s) in older rats.